The present invention relates to methods and apparatus for disposing of wastes, including solids, sludges, and liquids, and particularly to such methods and apparatus for waste disposal in underground formations.
With passage of the Resource Conservation and Recovery Act ("RCRA") in 1976, Congress envisioned the development of a regulatory program that would control "hazardous waste" from the "cradle"--the point of generation--to the "grave"--the point of ultimate disposal. RCRA defined the term "hazardous waste" as including any solid waste that, because of its quantity, concentration, or physical characteristics, may cause or significantly contribute to an increase in mortality or an increase in serious irreversible or incapacitating, reversible illness, or pose a substantial present or potential hazard to human health or the environment when improperly managed. Despite certain exceptions and later amendments, a broad definition of hazardous waste remains in RCRA. The term "toxic waste" is often defined somewhat differently, usually being applied to materials that cause immediate prolonged loss of normal body functions or death. In the present application, the terms "waste" and "hazardous waste" will be understood to include hazardous waste as defined in RCRA, as well as any toxic, radioactive, or other waste.
At present, there are four accepted approaches to the problem of hazardous waste: (1) source reduction, (2) recycling, (3) incineration, and (4) residual management. Source reduction imposes extensive regulatory constraints on generators of hazardous waste, thereby restricting progress in advanced consumer products such as television, telephones, energy, cars, plastics, etc. Significant progress in source reduction will come only from world-wide acceptance of such restrictions. Recycling involves "cleaning" and separating waste so that some of the waste constituents can be reused, often by their generator. While recycling reduces the waste volume, some residue of sludges and solids usually remains for disposal. Incineration requires modern combustion equipment coupled with proper air-emissions-control equipment. Although incineration can significantly reduce waste volume, it produces increased air emissions and an ash residue, which can be hazardous themselves.
Residual management is a modern version of land filling in which a "coffin" for the waste is carefully designed to control air emissions and soil and ground water contamination from leachate. The coffin is typically prepared by excavating from the surface to a stable area and backfilling with alternating layers of impermeable clay and waste; rubber or plastic liners are sometimes placed down before the first clay layer. After the coffin is filled, a final layer of clay permanently seals the coffin and wastes against water infiltration. Although much more acceptable than conventional landfilling, residual management produces some unavoidable air emissions, and because coffins typically reach depths of only about 60 feet and lie above aquifers, contamination can occur that is very difficult to repair.
U.S. Pat. No. 3,108,439 to Reynolds et al.; U.S. Pat. No. 3,331,206 to Osborne; U.S. Pat. No. 3,335,798 to Querio et al.; U.S. Pat. No. 3,374,633 to Brandt; and U.S. Pat. No. 3,513,100 to Stogner disclose another storage method involving cable tool or rotary drilling into suitable subterranean formations and pumping material, e.g., radioactive liquids or slurries, waste solids or sludges, or other liquids or gasses, into the formations. General aspects of well drilling are disclosed in the above-listed U.S. Patents and in U.S. Pat. No. 2,880,587 to Hendrix et al. and U.S. Pat. No. 3,064,957 to Jacoby. As described in the Stogner patent for example, a mixture of sharply angular solid waste particles and an aqueous cement are used to fracture and prop open an underground formation. Any incompressible fluid, like cement, can be used as a fracturing inducing fluid, but as described below, not all fracturing inducing fluids are suitable for permanent waste disposal.
Cement is a generic term representing many compositions and consistencies, but on average, pumpable cement has a viscosity between 20 and 30 centipoise. Such low viscosity is in marked contrast to the transport fluids described in more detail below, that develop viscosities as high as several hundred centipoise. This difference is one factor accounting for the large volumes of waste able to be handled by the present invention. In a static state, cement might hold as much as 2 pounds of radioactive solid waste per gallon if the waste were highly pulverized; as described further below, cross-linked hydroxypropyl guars, on the other hand, can hold 9 to 12 pounds per gallon and as much as 20 pounds or more per gallon down hole.
In addition, the concentration of waste able to be transported by cement and other low viscosity fluids is highly dependent on the flow rate. At low pump rates cement would carry only low concentrations of waste; at high pump rates, fluid velocity and turbulence would permit higher concentrations of waste to be transported. Thus, when used in waste disposal, cement would let the transported waste settle out whenever pumping was interrupted or the fluid flow velocity decreased As described further below, such a velocity decrease would typically occur when the fluid leaves the well and begins to enter the subterranean formation. Moreover, after a waste-cement mixture was injected into an underground formation, the waste would settle to the bottom of the fracture with little or no cement surrounding it. Such settling is particularly likely to occur for a radioactive waste such as uranium due to its high specific gravity.
Cement is also incompatible with certain wastes and has other undesirable properties. For example, cement eventually breaks down during prolonged exposure to nuclear radiation. Even for non-nuclear wastes, its permeability (typically about 50 millidarcys) would be much greater than a suitable waste disposal formation's permeability (about 0.1 millidarcy). Thus, cement would be unsuitable as a waste transport fluid because it would provide a leakage path through the formation instead of a permanent seal.
There are other significant differences between waste disposal in accordance with the present invention and typical oil and gas exploration practices. Conventional oilfield stimulation intentionally increases a formation's permeability, thereby permitting fluid transport to the well bore. A temporarily viscous transport fluid is used to fracture the formation and inject a proppant to keep the formation open, but such transport fluids are intended to break down to a very low viscosity and be removed from the well bore, thereby producing the minimum possible damage to the formation. Also, stimulation normally uses proppants that are very hard, well rounded substances, such as polished sand, to keep the fracture open and provide the highest possible fluid permeability in the formation. Such sand has a typical specific gravity of about 3, which is much lower than the specific gravity of about 18 of some radioactive wastes, and the low viscosity transport fluids used in conventional oilfield stimulation suffer from most of the same drawbacks for waste disposal as cement.
In contrast to conventional oilfield stimulation, the present invention advantageously provides for placement of hazardous wastes in a totally isolated environment with no chance of migration. In one aspect, the invention employs an incompressible, viscous transport fluid to fracture a selected underground formation, convey the waste underground, retain its viscosity or even harden with time in the formation, and have low to no permeability. A proppant would not be used to hold the formation open; the pressure of the waste-bearing transport fluid would keep the formation open during waste disposal. Thus, when the pressure was removed, the waste transported into the formation would be embedded therein due to its relative softness as the formation closed. Among the many advantages provided by the present invention is the expected much lower exposure to RCRA liability due to the known and controlled conditions of waste disposal sites in accordance with the present invention.